# Homework Help: Mechanical Energy

1. Mar 22, 2004

### kassy_in_the_sky

Several children, pretending they are playing in the jungle, suspend a rope from an overhead tree limb. A child of mass 40 kg running at 8.0m/s grabs the rope and swings off the level ground.

a) What maximum height does the child reach?
b) How fast would a 30 kg child have to run to reach the same height as the 40 kg child?

½mv1²+mgh1 = ½mv2²+mgh2

a) m=40kg, v1=8.0m/s, h1=0m, g=9.8m/s², v2=0m/s (at max height v=0)

h2=?

½mv1²+mgh1 = ½mv2²+mgh2

½mv1²= mgh2

h2 = 3.27m = 3m (is this correct?)

im really confused with B... coz i got 8m/s... it doesn't make sense to me how they can run at the same speed and achieve the same height??!!

2. Mar 22, 2004

Notice that the mass drops out of the equations entirely. So the mass has no significance to the problem.

Intuitively, if you increase the mass, the person has greater kinetic energy when he starts. But getting to the same height also requires more energy. It works out that the increases in energy are identical.

3. Mar 23, 2004

### kassy_in_the_sky

thanks! ^^

4. Mar 11, 2008

### ablatter

I believe that:

1. The initial inertial velocity is a convention between the different reference frames.

2. The total tensional energy of an isolated system is equal to zero.

3. The total energy of an isolated system is equal to zero.

Sincerely,

Antonio A. Blatter

5. Mar 11, 2008

### ZapperZ

Staff Emeritus
Unfortunately, you're responding to a question that's about 4 years old.

Zz.

6. Mar 12, 2008

### RetardedBastard

Hmm, why is that frowned upon? I mean, physics problems are timeless afterall :)